Method of vacuum forming an object using a flexible mold and an apparatus for vacuum forming an object

ABSTRACT

A method of vacuum forming an object includes heating a plastic sheet. After heating the plastic sheet, a vacuum is applied to pull the sheet against an outer surface of a flexible mold so that the plastic sheet has a formed shape that conforms to a contoured shape of the outer surface of the flexible mold. A rigid core is withdrawn from a cavity in the flexible mold. The flexible mold is then withdrawn from the plastic sheet by applying force to the flexible mold in a single direction, thereby causing flexing of the flexible mold past the undercut. An apparatus for vacuum forming a plastic sheet includes the flexible mold.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the benefit of priorityto U.S. patent application Ser. No. 14/557,516 filed on Dec. 2, 2014,which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present teachings generally include a method of vacuum forming anobject and an apparatus for vacuum forming an object.

BACKGROUND

Vacuum forming is a process by which a plastic sheet is heated and thenformed to the shape of a die by applying a vacuum to draw the sheetagainst the surface of the object. The die must then be withdrawn.Objects suitable for vacuum forming on a unitary rigid die haveheretofore been limited to those that do not have undercuts, as anundercut prevents withdrawal of the die from the formed sheet withoutdamage to the formed sheet. Alternatively, a complex, costly die withsliding die portions would be required to allow removal of the sheet.

SUMMARY

A method of vacuum forming an object includes heating a plastic sheet.After heating the plastic sheet, a vacuum is applied to conform pull theplastic sheet to a contoured shape of an outer surface of a flexiblemold so that the plastic sheet has a formed shape that includes anundercut. Typically, such an undercut would cause a die lock condition.Vacuum forming of an object to achieve such a formed shape was thereforenot an option. However, under the method disclosed herein, a rigid coreis withdrawn from a cavity in the flexible mold. The flexible mold isthen withdrawn from the plastic sheet by applying force to the flexiblemold in a single direction, thereby causing flexing of the flexible moldpast the undercut. Because the mold is flexible, the plastic sheet isformed to replicate the object, including the undercut. Thus, objectswith undercuts that previously required more costly processes, such as acomplex die assembly with slides, or injection molding, can instead bevacuum formed.

An apparatus for forming an object from a plastic sheet includes a moldhaving an inner surface defining a cavity and having an outer surfacethat has a contoured shape. A rigid core is configured to fit within thecavity. A vacuum source is positioned to vacuum form the plastic sheetto the outer surface of the mold to form the object having a formedshape, including an undercut, that conforms to the contoured shape ofthe mold. The formed shape includes an undercut. The mold is flexible,allowing the mold to be withdrawn from the plastic sheet past theundercut by applying force in a single direction after the rigid core iswithdrawn from the cavity.

The above features and advantages and other features and advantages ofthe present teachings are readily apparent from the following detaileddescription of the best modes for carrying out the present teachingswhen taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration in perspective view of an article tobe replicated that is a scale model of a pickup truck to be replicated.

FIG. 2 is a schematic illustration in perspective view of avacuum-formed plastic sheet replicating the scale model of FIG. 1.

FIG. 3 is a schematic illustration in another perspective view of thevacuum-formed plastic sheet of FIG. 2.

FIG. 4 is a schematic illustration in side view of a flexible moldsupported on a rigid core (shown in hidden lines) and on a base.

FIG. 5 is a schematic illustration in perspective view of the rigid coresupported on the base of FIG. 4.

FIG. 6 is a schematic illustration in perspective view of a containercontaining the article of FIG. 1 in mold material.

FIG. 7 is a schematic illustration in plan view of the container of FIG.6 with the base suspending the rigid core above a female mold formed inFIG. 6.

FIG. 8 is a schematic cross-sectional illustration taken at lines 8-8 inFIG. 7 of the rigid core suspended above the female mold formed in FIG.6 and with mold material poured in a gap to create the flexible mold ofFIG. 4.

FIG. 9 is a schematic illustration in perspective view of the plasticsheet clamped to an oven tray and an oven into which the tray isinserted and removed.

FIG. 10 is a schematic illustration in perspective view of the plasticsheet positioned above the mold assembly and base of FIG. 4 and with avacuum source connected to the base.

FIG. 11 is a schematic illustration in cross-sectional view of theplastic sheet vacuum formed to the flexible mold of FIG. 4 and showing acooling fan.

FIG. 12 is a schematic illustration in cross-sectional view of the rigidcore withdrawn from the flexible mold.

FIG. 13 is a schematic illustration in cross-sectional view of theflexible mold flexing to move past undercuts in the vacuum formed sheet.

FIG. 14 is a flow diagram of a method of vacuum forming an object.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers refer to likecomponents throughout the views, FIG. 1 shows an article 10 to bereplicated by a method 100 described herein and represented in the flowdiagram of FIG. 14. The article 10 is a scale model of a pickup truck.It should be appreciated that the article 10 is only one exampleembodiment of an article that can be replicated by the method 100, andthe method 100 can be applied to replicate other articles. Morespecifically, the method 100 is particularly useful for replicating anarticle that has an undercut that creates a die lock condition thatmakes a conventional vacuum forming process with a unitary rigid dieunusable. As used herein, an “undercut” is an angle of a portion of theouter surface of the formed object relative to a base on which a moldfor the object is mounted that makes it impossible to remove the moldfrom the formed object by applying only a force in a single direction tothe mold. For example, the article 10 of FIG. 1 has wheel wells 12 overtire and wheel assemblies 13, a front bumper 14 and a rear bumper 16,all of which result in undercuts in a vacuum formed plastic sheet 18formed over a flexible mold 20 of the article 10, as shown in FIG. 11.The object 10A formed from the plastic sheet 18 has undercuts 22A, 22Bat the replicated front and rear bumper 14A, 16A, respectively due toprotrusion of the bumpers 14, 16. Deeper undercuts 22C exist at thereplicated wheel wells 12A shown in the object 10A in FIG. 2 due toprotrusion of the vehicle body portion 17 above the wheel wells 12A. Asshown in FIGS. 2 and 3, the object 10A is formed from only the singleplastic sheet 18, and has a contoured outer surface 23A, and anopposite, contoured inner surface 23B that defines a cavity 23C.

As shown in FIG. 4, the flexible mold 20 is mounted to a base 24 in amanner described in greater detail herein. The base 24 has a basesurface 26 that is planar. The undercut 22A of the formed sheet 18(shown in FIG. 3) formed by a protruding portion 28A of the flexiblemold 20 forms a first angle A1 relative to the base surface 26. Becausethe undercut 22A is parallel with the base surface 26, the first angleA1 is 0 degrees when measured in a clockwise manner (left to right inFIG. 4) from the base surface 26, or 180 degrees when measured in acounterclockwise manner (right to left in FIG. 4) from the base surface26. A protruding portion 28B of the flexible mold 22 causes the undercut22B in the formed sheet 18 (shown in FIG. 3) that has a similar angleA2. The force F applied to remove the mold 20 from the cavity 23C formedin the object 10A in FIG. 13 is perpendicular to the base surface 26,such as a downward direction shown in FIGS. 4 and 13. The force F thusforms a second angle A3 relative to the base surface 26 that is 90degrees as shown in FIG. 4. In a traditional vacuum forming process witha single, integral, non-flexible (i.e., rigid) mold, withdrawal of sucha mold past the undercuts 22A. 22B, 22C would be impossible.

The method 100 enables the use of the flexible mold 20 that is of aflexible material such as silicone. As best shown in FIG. 12, theflexible mold 20 has an inner surface 30 defining a cavity 32, and hasan outer surface 34. The flexible mold 20 is formed as described hereinso that the outer surface 34 has a contoured shape that is complementaryto and mates with the formed shape of the object 10A. In other words,the outer surface 34 of the flexible mold 20 is in contact with and iscoextensive with the inner surface 23B of the object 10A (shown in FIG.3) prior to withdrawal of the flexible mold 20 from the object 10A. Theouter surface 34 is also substantially identical to the contoured shape35 of the outer surface of the article 10 (shown in FIG. 1).

FIG. 5 shows a rigid core 36 that is mounted on the base 24. The rigidcore 36 fits within the cavity 32 of the flexible mold 20 as is apparentin FIGS. 11 and 12. The rigid core 36 is shown with hidden lines in FIG.4, and is best shown in FIGS. 5 and 11. FIG. 5 shows that the base 24has a series of openings 40, only some of which are numbered in FIG. 5.Each of the openings 40 extends entirely through the thickness of thebase 24, from the surface 26 to an opposing surface 42. The openings 40extend around the rigid core 36 on the base 24. As shown in FIG. 11, acasing 44 creates a manifold 46 extending from a vacuum source (V) 48 tothe openings. Accordingly, the vacuum source 48 can apply a vacuum atthe surface 26 through the openings 40.

FIGS. 6-8 illustrate how the flexible mold 20 of FIG. 4 is made. Themethod 100 may include making the flexible mold 20 according to thesteps 102-114, or the method may begin with a pre-made flexible mold 20made according to the steps 102-114. First, step 102 includes creating afemale mold 50 that is then used to create the flexible mold 20, whichis a male mold. Step 102 includes sub-step 104, placing the article 10into a container 52. In sub-step 106, mold material 54 for the femalemold 50 is then poured or otherwise placed into the container 52 aroundthe article 10. FIG. 6 shows the mold material 54 poured over thearticle 10 to create the female mold 50. Clay 51 may be placed under thearticle, between the wheels 12 and from bumper 14 to bumper 16 toprevent mold material 54 from going under the article 10 when pouredinto the container 52. The mold material 54 may be a two-part siliconematerial. Once the mold material 54 is set, a cover 53 (shown in FIG. 8)is placed on top of the open container 52 in FIG. 6, and the container52 is inverted relative to its position during steps 102-106, as shownin FIGS. 7 and 8. A portion 55 of the container 52 that was previouslyon the bottom of the container 52, as shown in FIG. 6, is then removedto reveal the mold 50, as shown in FIG. 7. The article 10 and any clay51 thereunder is removed from the container 52 in sub-step 108, leavinga void 56 having a shape of the article 10, as partially shown in FIG.7. The female mold 50 is now complete.

Next, the method 100 proceeds to suspending the rigid core 36 above thefemale mold 50 in the container 52 in step 110 so that a gap 58 existsbetween the female mold 50 and the rigid core 36. FIG. 8 shows the gap58 already filled with mold material 60 for the flexible mold 20, perthe subsequent step 112, pouring mold material 60 for the flexible mold20 into the gap 58. The mold material 60 may be silicone. The resultingflexible mold 20 has an outer surface 34 (best shown in FIG. 4) with acontoured shape that is the same as the contoured shape of the outersurface 23A of the article 10 as a result of forming the female mold 50around the article 10 as described with respect to FIG. 6.

Next, in step 114, the flexible mold 20 is separated from the femalemold 50 by withdrawing the base 24 and rigid core 36 from the container52 and flexing the mold 20 out of the female mold 50. Next, in step 116,the flexible mold 20 is again placed on the rigid core 36 which issupported on the base 24, as best shown in FIGS. 4 and 11. Morespecifically, the rigid core 36 is placed in the cavity 32 of theflexible mold 20.

The plastic sheet 18 is then prepared for vacuum forming. The plasticsheet may be polyethylene terephthalate (PTEG), Acrylonitrile butadienestyrene (ABS), Polypropylene (PP), thermoplastic, or another polymericmaterial suitable for vacuum forming. In step 118, the plastic sheet 18is clamped to an oven tray 60. FIG. 9 shows an oven tray 60 with clamps62 that can be tightened to secure the plastic sheet 18. Optionally, theoven tray 60 could be sandwiched between an upper frame and a lowerframe, each of which may be rectangular similar to the over tray 60. Theclamps 62 could then clamp the frames together to secure the over trayand sheet 18 between the frames. Any suitable arrangement to secure thesheet 18 relative to the oven tray 60 can be used. The oven tray 60 withthe plastic sheet 18 clamped thereto is then moved at least partiallyinto an entrance 66 of an oven 64 in step 120 as indicated by thedirectional arrow B. The plastic sheet 18 is then heated in the oven 64in step 122. After a predetermined amount of time in the oven 64, theoven tray 60 and plastic sheet 18 are removed from the oven 64 in step124, as indicated by arrow C. The predetermined amount of time may beselected to ensure that the plastic sheet 18 reaches a predeterminedtemperature required for vacuum forming. A timer and/or temperaturesensors may be used to monitor heating of the sheet 18. Movement of theoven tray 60 into and out of the oven 64 could be done manually or couldbe automated. If the movement is automated, a robotic arm (not shown)could support and move the oven tray 60. A person skilled in the artwould readily understand the ability of a robotic arm to move the oventray 60 into and out of the oven 64.

With the plastic sheet 18 sufficiently heated, the flexible mold 20 isthen raised into contact with the plastic sheet 18 in step 126 by movingthe base 24 with the rigid core 36 and flexible mold 20 mounted thereonupward toward the sheet 18 as indicated by directional arrow D in FIG.10. The tray 60 is held in position either manually or by a robotic arm.Alternatively, the base 24 could remain stationary and the tray 60 couldbe moved toward the flexible mold 20 to place the plastic sheet 18 intocontact with the flexible mold 20. FIG. 11 show the plastic sheet 18 incontact with the flexible mold 20. The base 24 is continued to be movedtoward the sheet 18 until the sheet 18 pulls around the flexible mold20, and the vacuum 48 is applied in step 128. The vacuum 48 is in fluidcommunication with the plastic sheet 18 through the casing 44 and theseries of openings 40. Accordingly, the vacuum 48 helps to pull theplastic sheet 18 against the entire outer surface 34 of the flexiblemold 20 to conform the plastic sheet 18 to the contoured shape of theouter surface 34. The plastic sheet 18 is then cooled in step 130 toallow the formed shape of the plastic sheet 18 to become permanent.Cooling may be with an air source, such as a fan 70 shown in FIG. 11, orthe plastic sheet 18 may be cooled passively simply by waiting apredetermined amount of time until the plastic sheet 18 reaches apredetermined temperature, such as ambient temperature, as may bedetermined by temperature sensors. The formed shape of the plastic sheet18 is shown in FIG. 12.

Next, in step 132, the rigid core 36 is withdrawn from the cavity 32 inthe flexible mold 20 as shown by the directional arrow E in FIG. 12. Dueto the undercuts 22A, 22B, 22C (shown in FIG. 2), the flexible mold 20will tend to be retained by the plastic sheet 18 during removal of therigid core 36. With the rigid core 36 removed, the flexible mold 20 canmore easily flex inward relative to the inner surface 23B of the innercavity 23C of the plastic sheet 18 in step 134, in which the flexiblemold 20 is withdrawn from the plastic sheet 18. Withdrawing the flexiblemold 20 from the plastic sheet 18 in step 134 is accomplished byapplying force F to the flexible mold 20 which need be in only a singledirection as shown in FIG. 13. The force F is at 90 degrees in theembodiment shown, but may be at a different angle with respect to thebase surface 26 in other embodiments, dependent on the angle of theundercut(s) of the flexible mold of the particular article to bereplicated. Because the mold 20 is flexible, it flexes past theundercuts 22A, 22B, 22C of the plastic sheet 18. Arrows G1 and G2 inFIG. 13 represent the direction of inward movement of the protrudingportions 28A, 28B when the mold 20 flexes past the undercuts 22A, 22Bwhen the force F is applied in the direction shown.

Excess material of the plastic sheet 18 can then be trimmed from aperimeter P of the formed shape of the plastic sheet 18 in step 136. Theperimeter P is shown in FIG. 3. The excess material is represented bythe portions 74 in FIG. 13. With the object 10A now completed. Theflexible mold 20 can be reused to vacuum form additional objects 10Aaccording to the steps 116-136 of the method 100. The flexible mold 20is less expensive than rigid molds typically required for an injectionmolding process that would be used for forming an article withundercuts.

While the best modes for carrying out the many aspects of the presentteachings have been described in detail, those familiar with the art towhich these teachings relate will recognize various alternative aspectsfor practicing the present teachings that are within the scope of theappended claims.

1. An apparatus for forming an object from a plastic sheet comprising: a mold having an inner surface defining a cavity and having an outer surface; wherein the outer surface has a contoured shape; a rigid core configured to fit within the cavity; a vacuum source positioned to vacuum form the plastic sheet to the outer surface of the mold to form the object having a formed shape that conforms to the contoured shape of the mold, the formed shape including an undercut; and wherein the mold is flexible, allowing the mold to be withdrawn from the plastic sheet past the undercut by applying force in a single direction after the rigid core is withdrawn from the cavity.
 2. The apparatus of claim 1, further comprising: a base on which the rigid core is secured; wherein the base has a series of openings extending around the rigid core; and wherein the vacuum source is in fluid communication with the series of openings.
 3. The apparatus of claim 2, wherein the base has a surface on which the rigid core is secured; and wherein each opening of the series of openings extends through the base from the surface on which the rigid core is secured to an opposite surface of the base.
 4. The apparatus of claim 2, wherein the series of openings completely surround the rigid core.
 5. The apparatus of claim 2, wherein the base has a surface on which the rigid core is secured; wherein the undercut forms a first angle relative to the surface of the base; wherein the force is applied to the mold to withdraw the mold from the plastic sheet past the undercut at a second angle relative to the base; and wherein the second angle is different than the first angle.
 6. The apparatus of claim 5, wherein the second angle is a 90 degree angle such that the force is applied perpendicular to the base.
 7. The apparatus of claim 1, further comprising: a base having a base surface on which the rigid core is secured; and wherein the force is applied perpendicular to the base surface.
 8. The apparatus of claim 7, wherein the undercut is parallel to the base surface.
 9. The apparatus of claim 1, wherein a protruding portion of the mold forms the undercut.
 10. The apparatus of claim 9, wherein the object is a scale model of a vehicle having a wheel and a bumper, and the undercut is at the wheel or at the bumper.
 11. The apparatus of claim 1, wherein the mold is silicone.
 12. The apparatus of claim 1, wherein the outer surface of the flexible mold is configured to be coextensive with an inner surface of the object.
 13. The apparatus of claim 1, wherein an outer surface of the object is substantially identical to the contoured shape of the outer surface of the flexible mold.
 14. An apparatus for forming an object from a plastic sheet comprising: a mold having an inner surface defining a cavity and having an outer surface; wherein the outer surface has a contoured shape; a rigid core configured to fit within the cavity; a vacuum source positioned to vacuum form the plastic sheet to the outer surface of the mold to form the object having a formed shape that conforms to the contoured shape of the mold, the formed shape including an undercut; a base having a surface on which the rigid core is secured; wherein the base has a series of openings completely surrounding the rigid core; wherein each opening of the series of openings extends through the base from the surface on which the core is secured to an opposite surface of the base; wherein the vacuum source is in fluid communication with the series of openings; and wherein the mold is flexible, allowing the mold to be withdrawn from the plastic sheet past the undercut by applying force in a single direction after the rigid core is withdrawn from the cavity.
 15. The apparatus of claim 14, wherein the force is applied perpendicular to the base surface.
 16. The apparatus of claim 15, wherein the undercut is parallel to the base surface.
 17. The apparatus of claim 14, wherein a protruding portion of the mold forms the undercut.
 18. The apparatus of claim 17, wherein the object is a scale model of a vehicle having a wheel and a bumper, and the undercut is at the wheel or at the bumper.
 19. The apparatus of claim 14, wherein the outer surface of the flexible mold is configured to be coextensive with an inner surface of the object.
 20. The apparatus of claim 14, wherein an outer surface of the object is substantially identical to the contoured shape of the outer surface of the flexible mold. 